Heart rate variability: useful data, clear limits

Know which number you are using

HRV is different from resting heart rate. RMSSD summarises successive beat-interval differences using their squared values and is commonly expressed in milliseconds. SDNN is a different calculation; an app’s proprietary readiness score may not be a direct HRV measurement at all.

Laborde and colleagues describe how selected HRV indices can reflect cardiac vagal modulation. Recording length, body position, movement, breathing and signal artefacts affect interpretation. Numbers collected with different methods are not automatically comparable.

A consistent monitoring routine

Follow a repeatable recording procedure and the device instructions. Keep the time, position and method consistent; note unusual circumstances. Inspect trends together with sleep, symptoms, perceived effort and actual performance rather than reacting mechanically to each daily change.

Slower breathing during a reading can alter the result. A higher number after that manoeuvre does not prove improved training adaptation. Nor is a particular millisecond range a universal HYROX^® fitness standard.

What training research shows

Kiviniemi and colleagues randomised 26 healthy, moderately fit men to predefined running training, HRV-guided training or a control group for four weeks. The guided group improved maximal running velocity more than the predefined group; the change in peak oxygen uptake did not differ significantly between those groups.

This is evidence for a particular protocol and population. It is not proof that consumer readiness scores prevent overtraining, that every high-HRV day needs a hard session, or that a three-day reduction always requires a deload. The study’s measurement and decision rule should not be replaced with a different metric while retaining the same certainty.

When numbers disagree with symptoms

Use the result as one input. Persistent fatigue or deterioration in performance deserves attention even when a device looks reassuring. The overtraining consensus does not provide a single accepted diagnostic marker; HRV also cannot measure cortisol or diagnose illness.

Questions

What is a good HRV score?

There is no universal target across ages, metrics and devices. Compare a consistent personal series, with the measurement method and health context in mind.

Should a low reading automatically cancel training?

No. Check measurement quality and the wider situation. Persistent symptoms or reduced performance call for a broader review than a single score.

Does a high score guarantee readiness?

No. HRV does not certify muscle recovery, rule out illness or guarantee that a demanding session is appropriate.